The winner of the 2019 New York City marathon and the fastest marathoner in the world share a few things in common: They’re both from Kenya, they train together, and they wear Nike Vaporfly running shoes.

In fact, the $250 neon shoes (they come in bright green, pink, and orange) have been involved in nearly every major running milestone for the last three years.

But some runners and researchers think the footwear confers an unfair advantage. Both independent and Nike-sponsored studies have confirmed that the shoes increase athletes‘ energetic efficiency by 4% or more, which yields significant dividends in marathon-length distances. The shoes‘ foam and carbon-fiber sole is designed to ensure that less energy is lost in each footfall.

Pedestrians quickly make their way across a street in the Brooklyn borough of New York as runner Shura Kitata of Ethiopia passes close by during the New York City Marathon on November 3, 2019.Craig Ruttle/AP

„The runner runs the race, but the shoe enables him or her to run it faster for the same effort or ability,“ Geoff Burns, a kinesiology researcher and pro-runner, told Business Insider in an email. „So for two athletes of equal ability on race day, the one with the shoes is going to beat the one without the shoes.“

The Nike Vaporfly sole helps runners lose less energy per step

Jake Riley, an American runner who finished ninth in the 2019 Chicago marathon, has said the Nike Vaporfly 4% shoes feel like „running on trampolines.“

The secret is in the sole, which is designed to help runners get the most forward push for each stride — or, in Burns‘ words, to run faster for the same „energy expenditure.“ The soles consist of a foam layer and carbon-fiber plate fused together.

In addition to protecting our legs from the impact of striking the ground, running shoes store and release energy to propel us forward. The midsole acts like a spring, compressing when a runner lands, storing the energy from that foot strike, and expanding again to return that stored energy into the ground to push them forward.

Not all of that stored energy gets returned with each footfall, though — some dissipates as heat. But the Vaporfly’s design minimizes that amount of lost energy, giving the runner more bang for the buck.

Traditional running shoes generally use ethylene vinyl-acetate foam, which returns about 65% of the energy you put into it, according to Burns. The Vaporfly, by contrast, uses a new type of foam called Pebax, which is about 87% efficient. (The patent is owned by a French chemical company called Arkema.) The addition of the carbon-fiber plate helps the Pebax foam compress and expand quickly.

„Otherwise it would be like a marshmallow,“ Burns said.

Kyle Barnes, a movement scientist who authored a study about the Vaporfly shoes in February, told Business Insider that the carbon-fiber plate is curved under the front of the shoes, which also makes a big difference. That curvature, he said, helps quickly rock a runner from their heels to their toes as they land and push off again.

„As soon as you put the shoes on, you have this ‚Aha!‘ moment in which you know these are different than anything you’ve put on before,“ Barnes said. „I have several pairs.“

Over 26.2 miles, 4% more efficiency is a lot

Barnes‘ February study, which he conducted independently of Nike, found that the Vaporfly shoes improved an athlete’s running economy by 4.2% compared to Adidas Adizero Adios 3 shoes.

Another independent study looked at early Vaporfly models in November 2017 and reached the same conclusion: „The prototype shoes lowered the energetic cost of running by 4% on average,“ the researchers wrote.

Over marathon-length distances, that 4% can mean a lot — a person running a 2-hour-10-minute marathon would see a 3.5-minute improvement in speed. For athletes like Kamworor and Riley, that could be the difference between setting a world record and falling short.

The authors of the 2017 study even predicted at the time that „with these shoes, top athletes could run substantially faster and achieve the first sub-2-hour marathon.“

Kenya\'s Eliud Kipchoge, the marathon world record holder, runs with his pacesetters during his attempt to run a marathon in under two hours in Vienna, Austria, October 12, 2019.Leonhard Foeger/Reuters

Kipchoge was wearing a prototype of the Nike Vaporfly, called AlphaFly, that hasn’t hit the market yet. His phalanx of pace-setters that kept him on track were also wearing Vaporflys.

The Nike Vaporfly has already taken over the marathon world

Kipchoge’s sub-2-hour marathon didn’t set a new world record because of IAAF rules, but he broke the world marathon record in Berlin in 2018 — wearing Vaporflys.

In September, when Kamworor set the record for fastest half-marathon (58 minutes and 1 second), he was also wearing Vaporflys.

The shoes have plenty of other fans as well. In 2019, runners wearing Vaporflys claimed 31 out of the 36 male and female podium spots in the six biggest marathons around the world. The three medalists in the men’s marathon at the 2016 Summer Olympics all wore a Vaporfly prototype.

In this year’s Chicago marathon, the top 10 male finishers were wearing the shoes, too. And female runner Brigid Kosgei, who broke the world record in the 2019 Chicago race with a time of 2 hours, 14 minutes, and 4 seconds, wore Vaporflys to accomplish the feat.

Kenya\'s Brigid Kosgei celebrates setting a new world record to win the women\'s event in the 2019 Chicago marathon on October 13, 2019.Mike Segar/Reuters

„It’s hard to know what we’re actually watching in some respects — is it the technology, or the athletes?“ Barnes said. „I know you have to be an exceptional human being to come close to these achievements, but the jumps we’re seeing is the technology.“

‚Technology doping‘

The International Association of Athletics Federations (IAAF), which governs most international track and field events, has yet to make any move to ban or regulate the sneakers. Currently, IAAF rules say shoes can’t confer an „unfair assistance or advantage“ and have to be „reasonably available“ to everyone. But the organization doesn’t define those standards more specifically.

Barnes compared the Vaporfly trend to the swimming races at the 2008 Beijing Olympics, in which competitors set 25 world records. That year, 98% of the Olympic field was wearing Speedo’s LZR Racer swimsuit, a full-body sleeve of polyurethane designed to mimic shark skin.

„They called it ‚technology doping,'“ Barnes said. „Even non-Speedo athletes had to switch and wear the suit or there was no chance of them competing. We’re not quite at that point yet with the Vaporflys, but close.“

In 2009, the International Swimming Federation banned all full-body polyurethane suits, including the LZR.

But when asked last year whether the Nike Vaporfly 4% shoes confer an advantage, Kamworor said, „I don’t think that the shoe is a factor.“

„Provided that you are prepared, that you are training hard, you can run with any kind of shoe. So the shoe is not a disadvantage to other people,“ he said in a press conference.

Kipchoge 2016 rio marathonLucy Nicholson/Reuters

But Burns and Barnes both said they don’t think the recent running records, as well as Kipchoge’s sub-2-hour marathon, would have been possible in different footwear.

Nor does Mary Wittenberg, the former New York Road Runners president.

„I actually think we’re going to have asterisks on all the results that are like AV and BV — Before Vaporfly,“ she told The Wall Street Journal before the New York marathon.

Should the IAAF regulate running shoes?

Many runners sponsored by other shoe companies would like to see the IAAF lay down additional rules about permissible shoes. Sara Hall, an Asics-sponsored runner and Ryan Hall’s wife, told Outside Online that because of the shoes, „it’s hard to really just celebrate performances at face value right now.“

„I think it would help to have some limits, just like other sports have, like swimming, or triathlon, or cycling,“ she added. „They all have limits of the gear. So I think that would help create more of an even playing field.“

According to Burns, one option for the IAAF could be to limit how thick a shoe’s midsole can be — he proposed this in an October paper.

„As we allow that height limit to go greater and greater, more and more of that energy recycling is being done by the shoe, so the performances are less and less human,“ he said.

Current Vaporfly models have 1.4 inch-thick soles, whereas midsoles of other racing shoes generally hover around 1 inch, Burns noted. So in his paper, he suggested capping thickness at 1 inch.

That kind of limit, Burns said, „would define the space on a runner that can be a ’shoe,‘ and allow companies to innovate within that space.“

But such a rule would disqualify Vaporflys.

„I think we need to draw a line,“ Burns said. “ If I got to draw the line, I would draw it so as to preserve the performances of the past 40 years at the cost of the last three years.“